Using the idea that nuclear magnetism of protons in water persist for the greater part of a second, we are developing methods for tracing and measuring blood flow that work entirely thru electromagnetic fields. Protons in water of the blood polarized by a strong static magnetic field retain the polarization and thereby produce a stronger signal in the downstream blood umpolarized blood. In the tracer mode an oscillating field is used to erase the polarization at a specific point to discriminate the proportion of blood that passed through the erasing field. Various locations of the polarizing and erasing field are used to provide information about blood flow in various organs. A specific aim is to provide an instrumental basis for measuring intracranial blood flow and distribution to facilitate studies of intracranial atherosclerosis and stroke. BIBLIOGRAPHIC REFERENCES: Sances, A., Jr., Battocletti, J. H., Halbach, R. E., Larson, S. J., Evans, S. M., Bowman, R. L., and Kudravcev V.: "Limb blood flow measurement by NMR", Digests of XI Int'l. Conf. on Medicine and Biology in Engineering, Ottawa, Canada, August 1976, pp. 430-431. Malfertheiner, P., Sances, A., Jr., Battocletti, J. H., Larson, S. J., Halbach, R. E., Evans, S. M., Bowman, R. L., Fegiz, G., and Ray, L.: "NMR (nuclear magnetic resonance) a non-intrusive technique for measurement of blood flow", Surgery in Italy 6(3): 186-194, September 1976.